Recently, with the development of portable electronic devices such as notebook-sized personal computers, cellular phones, etc., there is a need for high-density and high-capacity secondary batteries. Typical secondary batteries that have been used are nonaqueous electrolyte secondary batteries in which lithium cobaltate is used for a positive electrode active material while a carbon-based material is used for a negative electrode active material.
There is a need for further increases in density and capacity of batteries, which, however, can result in the possibility of heat generation. For instance, when an internal short circuit is caused through the breakage of a battery due to the misuse thereof, the penetration of a metal body, etc., the battery that has an increased density and capacity is expected to generate heat. When an internal short circuit occurs, electrons flow through the short-circuited part locally to generate Joule heat. This heat causes a separator to shrink and thereby the area of the short-circuited part between the positive electrode and the negative electrode expands. Further, the positive electrode active material that has melted due to the Joule heat comes into contact with the negative electrode and thereby oxygen contained in the positive electrode active material causes an oxidation reaction with active lithium contained in the negative electrode active material. This may result in abrupt heat generation. Such abrupt heat generation actually can occur in batteries having an increased density and capacity. In order to solve this problem, a nonaqueous secondary battery has been proposed that includes a collector whose surfaces have increased electric resistance (JP10 (1998)-199574A). This secondary battery includes a resistive layer that is formed on each surface of a positive electrode or negative electrode collector and that has a higher resistance than that of the collector.
The increase in resistance of the collector surface, however, results in an increase in internal resistance of the battery and thereby has adverse effects on the characteristics such as the battery capacity, operating voltage, cycle characteristics, etc. Accordingly, there is a demand for new nonaqueous electrolyte secondary batteries that can prevent abrupt heat generation from being caused when an internal short circuit occurs, without impairing the battery characteristics.